The present invention relates to a sheet delivery mechanism of an image forming apparatus, such as a printer or a copying machine, for discharging sheets of printing media carrying printed images as well as to an image forming apparatus incorporating the sheet delivery mechanism.
When an image forming apparatus for producing printed images on sheets of paper outputs multiple copies of printed sheets onto a sheet delivery tray provided outside a housing of the apparatus, for instance, it is usually difficult to discern boundaries between the individual copies and, therefore, a user needs to sort the individual copies upon completion of image forming operation. When producing two copies of a 10-page document by the image forming apparatus, for example, the user has to find out a boundary between the first and second copies, or the boundary between a last page of the first copy and a first page of the second copy, and separate the two copies from each other.
There are several known types of sheet delivery devices applicable to conventional image forming apparatuses for realizing efficient sorting operation. These sheet delivery devices are designed to selectively deliver multiple copies of printed sheets in different ways or at different (offset) locations by varying sheet delivery positions for easy sorting. Four specific approaches employed in these sheet delivery devices are as follows.
A first approach proposed in Japanese Laid-open Patent Publication No. H11-199124, for example, is to feed printing paper of the same size in different orientations (portrait and landscape) for every other copy of a document and output printed sheets in the same orientations. Specifically, sheets for printing one copy are fed in such a way that a short side of each sheet goes first and sheets for printing next copy are fed in such a way that a long side of each sheet goes first. As the printed sheets are discharged in the same orientations, individual copies can be easily distinguished.
A second approach disclosed in Japanese Laid-open Patent Publication No. 2000-086056, for example, employs an offset tray. Although printed sheets are discharged from a fixed sheet output position, the offset tray is shifted (offset) to different positions so that the printed sheets are delivered to different locations for easy sorting.
A third approach proposed in Japanese Laid-open Patent Publication No. H05-186121, for example, includes a pair of paper output rollers individually mounted on two shafts and a differential gear mechanism provided between the two shafts. While the two paper output rollers nip a printed sheet for discharging it, a difference is produced between rotating loads of the two paper output rollers. The differential gear mechanism produces a difference in rotating speed between the two paper output rollers so that multiple copies of printed sheets are output to different sheet delivery positions for easy sorting.
A fourth approach shown in Japanese Laid-open Patent Publication No. H08-208091, for example, includes a driving roller assembly and pinch roller assemblies for discharging printed sheets. While a printed sheet to be discharged is nipped between the roller member and the pinch roller assemblies, the driving roller assembly is shifted in its axial direction. As the nipped sheet pulled by the driving roller assembly is also shifted in the axial direction of the driving roller assembly, the sheet delivery position of each sheet is varied to facilitate a sorting job.
An image forming apparatus recently developed is a so-called front access type shown in FIG. 7, which is intended to achieve compactness in design. Referring to FIG. 7, the front-access-type image forming apparatus includes an image scanning section 160 located at an upper part, a paper feed section 170 located at a lower part for feeding sheets of paper used for image forming, and an image forming section 180 disposed between the image scanning section 160 and the paper feed section 170. The image scanning section 160, the image forming section 180 and the paper feed section 170 are arranged generally in a U shape in cross section. A sheet delivery section 190 to which each sheet carrying a printed image is output is provided in a space between the image scanning section 160 and the paper feed section 170. To meet an increasing demand for advanced features, this type of image forming apparatus incorporates a duplex (double-sided) image-forming function which is realized by a switchback paper transfer method instead of a normally used intermediate tray method. In the switchback paper transfer method, a sheet of paper is reversed by transferring the sheet in a direction opposite to an ordinary sheet transport direction through a paper transfer path S′ by means of a pair of paper output rollers 191 immediately after an image has been formed on one side of the sheet.
For the front-access-type image forming apparatus, it is not desirable to employ the aforementioned first approach of Japanese Laid-open Patent Publication No. H11-199124. This is because it is necessary to provide multiple paper cassettes for each paper size to feed the printing paper in different orientations and this makes it difficult to achieve compactness of the apparatus. The aforementioned second approach of Japanese Laid-open Patent Publication No. 2000-086056 is not desirable for the front-access-type image forming apparatus either, because it is difficult to accommodate a movable offset tray in the limited space of the sheet delivery section 190. Accordingly, the aforementioned third and fourth approaches shown in Japanese Laid-open Patent Publication Nos. H05-186121 and H08-208091, respectively, seem to be suited to the front-access-type image forming apparatus, because arrangements of these approaches do not require a large space.
The arrangement of Japanese Laid-open Patent Publication No. H05-186121 is not so preferable, however. Since a sheet nipped by the two paper output rollers turning at different rotating speeds is discharged as if along a parabola and nipped portions of the sheet are advanced at different speeds, an undesired load, warpage or slack is likely to occur in the sheet.
In the arrangement of Japanese Laid-open Patent Publication No. H08-208091, a driving force transmission device which is connected to a prime mover and transmits a driving force for turning the driving roller assembly needs to be shifted together with the driving roller assembly for design-related reasons. Therefore, a large space is needed to allow the shifting of the driving roller assembly in its axial direction and, as a consequence, it is difficult to provide multiple sheet delivery positions offset from a reference position. Furthermore, if a gear mechanism is used for connecting the driving force transmission device to the prime mover, gears of the mechanism would gradually wear due to friction caused by the shifting of the driving roller assembly in its axial direction and slip over one another, eventually becoming difficult to precisely transmit the driving force to the driving roller assembly.
In addition, since the driving force transmission device shifts the driving roller assembly only without shifting the pinch roller assemblies, the sheet nipped between the roller member and the pinch roller assemblies is dragged along the pinch roller assemblies so that the sheet shifts together with the driving roller assembly. Thus, when images are to be formed on both sides of the sheet, the image formed on one side is likely to be smeared due to friction between the sheet and the pinch roller assemblies.
If a mechanism including a cam and a spring is used for connecting the driving force transmission device to the prime mover, an elastic force of the spring for shifting the driving roller assembly in its axial direction would gradually decrease due to deterioration over time and it might become impossible for the driving force transmission device to exactly shift the driving roller assembly to specific positions.
For the aforementioned front-access-type image forming apparatus which is intended to achieve compactness in design, it is essential that the paper output rollers be controlled with increased accuracy since the apparatus employs the duplex image-forming function based on the switchback paper transfer method. Specifically, the paper output rollers should be repeatedly shifted in their axial direction with increased positioning accuracy along the axial direction and with increased accuracy of driving force transmission from the prime mover to output multiple copies of printed sheets at specific offset locations. When a sheet carrying an image already formed on one side is switched back for performing a duplex image forming job, for example, a paper jam or an oblique paper feed may occur while the sheet is being transferred for forming an image on a reverse side if the paper output rollers 191 are not correctly positioned or the driving force is not properly transmitted to turn the paper output rollers 191 in their forward and reverse turning directions. These paper feed problems could result in such a problem that the images formed on both sides of the sheet are misregistered from each other. When the arrangement of Japanese Laid-open Patent Publication No. H08-208091 is employed in the front-access-type image forming apparatus, the aforementioned problem is likely to occur in the duplex image forming job.
As stated earlier, one of pending problems of the conventional image forming apparatuses is that it is difficult to distinguish boundaries between multiple copies of a multiple-page document. More specifically, it is difficult for a user to discern a boundary between the last page of the first copy and the first page of the second copy, for example, so that the user has to find out the boundary between the first and second copies and separate the individual copies from each other.
To overcome this inconvenience, the prior art proposes various kinds of sheet delivery mechanisms featuring a shifter function which enables a user to distinctly discern boundaries between multiple copies (prints) of a multiple-page document. Arrangements for realizing the shifter function are roughly divided into three types.
A first arrangement is to feed printing paper in different orientations (portrait and landscape) and rotate printed images clockwise and counterclockwise by 90 degrees for every other copy of a document as proposed in Japanese Laid-open Patent Publication No. H11-199124, for example.
A second arrangement is to use a movable offset tray which is shifted (offset) to different positions when receiving multiple copies of printed sheets ejected from a fixed sheet output position as proposed in Japanese Laid-open Patent Publication No. 2000-086056, for example.
A third arrangement is to vary the sheet delivery position by shifting (offsetting) printed sheets being discharged by means of a driving roller assembly provided in a sheet delivery section as proposed in Japanese Laid-open Patent Publication No. H08-208091, for example.
As already mentioned, the aforementioned front-access-type image forming apparatus has the duplex image-forming function to meet the demand for advanced features. To achieve compact design of the apparatus, the duplex image-forming function is realized by the switchback paper transfer method, in which the sheet is reversed for performing each duplex image forming job immediately after an image has been formed on one side of the sheet, and not by the ordinary intermediate tray method.
It is not desirable to use the aforementioned first arrangement for implementing the shifter function in the front-access-type image forming apparatus. This is because it is necessary to provide multiple paper cassettes for each paper size to feed the printing paper in different orientations in the first arrangement and this makes it difficult to achieve compactness of the apparatus. It is not desirable to use the aforementioned second arrangement for implementing the shifter function in the front-access-type image forming apparatus either. This is because it is quite difficult to accommodate a movable sheet delivery tray (offset tray) in a limited space available in a central empty space of a generally U-shaped structure (in cross section) of the apparatus.
The aforementioned third arrangement is seemingly suited to the front-access-type image forming apparatus. This is because a sheet delivery tray may be held at a fixed position and there is no significant difficulty in reducing the size of the apparatus in the third arrangement, in which the driving roller assembly located in the sheet delivery section offsets the printed sheets to vary the sheet delivery position. When the third arrangement is employed in the image forming apparatus, however, these arises a problem related to ease of stacking the printed sheets output to offset delivery positions. If the image forming apparatus can not neatly stack the printed sheets delivered to offset positions, it may become impossible for the user to easily separate individual copies.